The present invention relates to an apparatus for manufacturing magnetic random access memory (MRAM) devices, and more particularly to an apparatus for orienting magnetization directions of magnetic layers in memory elements of MRAM devices.
Spin transfer torque magnetic random access memory (STT-MRAM) is a new class of non-volatile memory, which can retain the stored information when powered off. An STT-MRAM device normally comprises an array of memory cells, each of which includes at least a magnetic memory element and a selection transistor coupled in series between appropriate electrodes. Upon application of an appropriate write current to the magnetic memory element, the electrical resistance of the magnetic memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
The magnetic memory element typically includes a magnetic reference layer and a magnetic free layer with an insulating tunnel barrier or junction layer interposed therebetween, thereby collectively forming a magnetic tunneling junction (MTJ). Upon the application of an appropriate write current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction of the magnetic reference layer. The insulating tunnel junction layer is normally made of a dielectric material with a thickness ranging from a few to a few tens of angstroms. When the magnetization directions of the magnetic free and reference layers are substantially parallel, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistivity of the MTJ. Conversely, the electrical resistivity of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel. Accordingly, the stored logic in the magnetic memory element can be switched by changing the magnetization direction of the magnetic free layer.
Based on the relative orientation between the magnetic layers and the magnetization directions thereof, an MTJ can be classified into one of two types: in-plane MTJ, the magnetization directions of which lie substantially within planes parallel to the layer plane, or perpendicular MTJ, the magnetization directions of which are substantially perpendicular to the layer plane.
The magnetization directions of various magnetic layers in the MTJ, such as the magnetic reference layer and free layer, need to be initialized in proper directions in order for the MTJ memory element to function properly. For example, the magnetization directions of the magnetic free and reference layers may be oriented to be anti-parallel to each other, thereby setting the MTJ memory element to the high resistance state. For the foregoing reasons, there is a need for an apparatus that can align the magnetization directions of various magnetic layers in MTJ memory elements and that can be inexpensively manufactured.